text: In comparison with its better-known archaeal homolog bacteriorhodopsin, most of the active site residues of known importance to the bacteriorhodopsin mechanism are conserved in proteorhodopsin.  Sequence similarity is not significantly conserved however, from either halo- or bacterio- rhodopsin. Homologues of the active site residues Arg82, Asp85 (the primary proton acceptor), Asp212 and Lys216 (the retinal Schiff base binding site) in bacteriorhodopsin are conserved as Arg94, Asp97, Asp227 and Lys231 in proteorhodopsin. However, in proteorhodopsin, there are no carboxylic acid residues directly homologous to Glu194 or Glu204 of bacteriorhodopsin (or Glu 108 and 204 depending on the bacRhodopsin variant), which are thought to be involved in the proton release pathway at the extracellular surface.  However, Asp97 and Arg94 may replace this functionality without the close residue proximity as in bacteriorhodopsin.  The department of chemistry at Syracuse University decisively showed Asp97 cannot be the proton release group as the release happened at forcing conditions under which the aspartic acid group remained protonated.